Retrofitting log house with light straw clay

Hello everybody! Happy to have found this forum - what a great online community!

I'm starting a building project, renovating an old house with 15cm thick, hand-hewn log walls. I'm thinking of bringing up the insulation value by adding insulation to the exterior wall.

As I was looking for something natural, affordable and DIY I found light straw clay insulation. My plan is to build a framework against the outer wall just as one would when insulating with regular batt insulation.
The exterior finish will be wooden cladding / siding with a small air gap 3-5cm between the straw clay.

Your wall would have to be very thick, 30-60 cm, to match current insulation standards (read a study on it here). How would that affect your, windows, doors and roof line?

Herman Aus

Posts: 3

posted 5 years ago

Hey Andrew,

Very helpful study. Thanks for that.

Including the log wall and the exterior cladding, the maximum thickness of the wall I could allow is 35cm (14 inches). This would allow for a maximum of 15cm (6 inches) of straw clay.
The windows and the doors would be moved outwards accordingly but the roof overhand would have to remain constant.

Using the table from the study you provided and adding the R-value of the log wall, the total R-value would be about 18-20.
I think the recommended insulation here in Estonia is similar to that of Alaska - R-25 or so.

The lesser insulation value of straw clay is definitely a minus.
The affordability and local availability is a major plus though.

I build traditional timber frames and log structures. If I want more insulation I just expand the wall truss system I use. Most of my walls start at 250 mm and go up to 600 mm in thickness,or thicker. You have been looking at only the R factor, which is what everyone focuses on and the forget about thermal mass element of a structure. That is why a log cabin can have R 11 and still feel warm and a stone house with 300 mm walls and a 2" foam outer layer can be very warm to hot, holding its temp for sometime. I'm not sure what method you will use for creating the extra thickness, but you should consider a wall truss system, it may serve you well.

Regards. jay

Andrew Parker

pollinator

Posts: 514

Location: Salt Lake Valley, Utah, hardiness zone 6b/7a

4

posted 5 years ago

Given your wall thickness constraint, I would recommend you consider foam insulation panels. It is not as natural as straw clay, but it would be appropriate.

As was determined in an earlier thread on retrofitting straw bales to an existing structure, wall width can often be an obstacle that cannot be overcome.

I was thinking of using something similar to a wall truss system to hold the insulation. That also makes attaching the wooden cladding easy.

Another option I'm considering is using thatch / reed in between the wall trusses. I have seen some old log walled barns here that have their exterior walls insulated with thatch. It's a great local resource but not the cheapest, which is why I'm thinking of other grassy alternatives.

There is a local company that even produces convenient, 5cm thick reed panels that can be used exactly like regular insulation panels but they are rather expensive.

Trying to find the best balance of natural, efficiency and affordability.

Another important consideration is the work involved with a particular method. Installing thatch costs more but would be quicker to install than clay straw and the overall insulation would be greater.

Thanks Andrew for the great resources. They have been a great help.

Foam insulation could work perfectly indeed - thin, warm and quick to install - but I'm too much of a purist to bring myself to using it.

Thank for the input everyone, appreciate the help.

Herman

Andrew Parker

pollinator

Posts: 514

Location: Salt Lake Valley, Utah, hardiness zone 6b/7a

4

posted 5 years ago

I think that with reeds you will run into the same wall thickness issue. You must compensate for lower insulative value with greater thickness. Any of the light clay solutions should work great but you will need at least a 30 cm thick wall. Adding the inner and outer covering puts it in the 35-40 cm range -- for new construction.

You could also consider MgA cement foam. You could pour it in the 15 cm gap between the siding and the existing wall.

Good luck with whatever you decide, and if you stumble and use foam panels, I won't tell anyone.

I agree with Andrew and Jay and would add that Air-tightness is the most important factor to consider. Air tight building envelopes allow Insulation and thermal mass to do their job. The thickest, highest R value is useless if there are air leaks all around it.

I understand your purist attitude. Just remember that its the long term energy costs resulting from the building envelope that is the biggest source of most homes and buildings environmental impact. Foam uses oil and is "unnatural" in the eyes of many but it can dramatically reduce the bigger source of energy and environmental costs for many years to come. Foam by volume is 95% air and extremely durable.

Mineral wool is another option for exterior insulative sheathing.

"If you want to save the environment, build a city worth living in." - Wendell Berry

In my training with light clay there was a technique with higher R-value used under roofs, that could be adapted for your situation. basically, the straw is tossed in clay slip, then piled loosely into the cavity. More air spaces means higher R-value. Still fire-resistant. Of course you would want your outer sheathing to be pretty tight. And then deal with ventilation for your interior. Another helpful fact - super-thick light clay walls tend not to dry. I was told the max should be 8".
If you put light clay on the inside of the walls, you will have some good heat storage. Of course you'll lose living space... but if it's feasible, you would skip the exterior finish and have a lovely interior wall. (maybe not more lovely than those logs though)

Good luck!

Funny that it takes those bribes of free books to get me onto the site. Probably because I don't have my land yet.